1. Field of the Invention
This invention relates to an optical-fiber incorporated longer-sized subaqueous unit in which optical fibers are combined with a power cable, water supply tube, or the like to be laid down on the bottom of water.
2. Description of the Prior Art
A conventional counterpart of the invention, for example, an optical-fiber incorporated submarine power cable is constructed in such a manner that a plastic sheath made of polyethylene, vinyl chloride, or the like having on its periphery a spiral or lengthwise groove is provided on the periphery of a single-core or three-core cable, wherein an optical fiber is embedded into the groove and armoring iron wires are further provided outside the plastic sheath, or alternatively that an optical fiber cable is provided in the form of spiral or SZ strand on the periphery of a power cable, whereon a plastic sheath and, further outside thereof, armoring iron wires are provided. Commonly, in the case of a three-core cable, an optical fiber is inserted into idle spaces between the cable cores for composition.
However, the optical fiber is weak in mechanical strength in general, buckling being critical therefor. Moreover, local bending or so-called microbend causes the transmission characteristic of an optical fiber to be deteriorated remarkably.
In some cases where an optical fiber is limited in length due to the bobbin of a rolling device or the like so that the optical fiber cannot be combined with a power cable in a series of length, the optical fiber needs to be joined with another. Otherwise, when power cables are involved in factory joining, it is also required for optical fibers to be joined at the same time.
Thus, it is necessary to accommodate the joining margins on a power cable or all along within the groove on a plastic sheath with optical fibers having been joined and permitting the incorporation thereof without any microbend. This technique has been the largest bottleneck in manufacturing optical fiber units. That is, optical fibers, when joined, are required to be away from the power cable main body. The marginal portion for this purpose involves a most difficulty to be accommodated without deteriorating the performance of the optical fiber. Further, when a plurality of optical fibers are used, different margin lengths thereof will almost prohibit them from being accommodated.